Electrical precipitation apparatus



May 22, 1951 C. W. J. HEDBERGy ELECTRICAL PRECIPITATION vPPARATUS FiledSept. 24, 1947 a. 6 25 24. 23 i i lf K+ f :i |l||| il 2/ lin III lll l lll l s!!! ,/7A /8 n Il f l .egggsm n L' :z /2 l U i I l I I o o n o l:fl Il ,3 l' Il' 9 11g g I il S i Il c o a 0 l; :1:: /4

C i s il /6 ll E z =f i i cvcLo/vs sEPAnA ons i i 2 n `27 2 7 ln/UMH@/0' [IAR LJWJHEDE'ERE a: '1a/Mv@ Patented May 22, 1951 ELECTRICALPRECIPITATION APPARATUS Carl W. J. Hedberg, Bound Brook, N. J., assignerto Research Corporation, New York, N. Y., a corporation of New YorkApplication September 24, 1947, Serial No. 7 75,804

9 Claims.

This invention relates to electrical precipitation apparatus and moreparticularly to such apparatus of relatively large size employing aplurality of precipitation chambers to which gas to be cleaned isdelivered from a plenum chamber.

The invention is particularly useful in controlling the loss of dustfrom precipitators when the electrodes are rapped for removal of theprecipitate from the collecting electrodes. This problem is especiallytroublesome with large precipitators handling large gas volumes such asare usually employed for removal of fly ash from the combustion gasesfrom large pulverized coal fired boilers. Ordinarily these boilersoperate continuously for long periods of time and it is not possible tointerrupt the gas flow through the precipitator unit employed forcollection of the ash either for rapping of the electrodes or for otherservicing operations.

In normal operation much of the precipitated dust falls from theelectrodes naturally into the hoppers below. However, there is aresidual accumulation of dust on the electrodes which necessitates thatthese be rapped or vibrated to loosen the residue and cause it to fallinto the hopper. When this operation is carried out with normal gasdow-ing, there is an unusually heavy discharge of dust from the stack,the maximum density of which may reach 3 to 5 times the density of ashin the gases normally coming to the precipitator. The frequency andduration of the vibrating operation will Vary over a considerable range.Frequency may be from once each 1/2 hour to once in 24 hours, and theduration of the vibration may run from approximately 2G seconds up to 2minutes. While the daily less of dust during this period may be arelatively small proportion of the total Zfi-hour load entering theprecipitator, the contrast in stack appearance resulting from therapping operation is objectionable in many installations.

In prior art constructions, dampers have been used either directly atthe inlet or outlet of the precipitator. Since most of theseprecipitators are very large, it has been found impractical with thisdamper location to redistribute the gas flow uniformly over the otherunits of the precipitator when one unit is dampered off for rapping.Usually this operation results in a very great increase in gasvelocities through the unit or units adjacent to that one which is beingrapped with relatively little added flow to the units further removed.As a result, the very great increase in velocity through an adjacentunit causes very high loss of dust from the unit even though it is notbeing rapped.

I have found that by inserting in the gas stream owing to eachprecipitation chamber means providing a high pressure drop, distributionoi the gas to remaining units is greatly facilitated.

It is a principal object of the invention to provide in electricalprecipitation apparatus of the type referred to above, meanssubstantially equalizing gas flow to a plurality of precipitationchambers arranged in parallel when one or more of the chambers isblanked off for electrode rapping or servicing.

A further object is to provide in an electrical precipitator forremoving fly ash from furnace gases, means accomplishing the primaryobject of the invention While retaining the inherent advantages of heatexchange, as provided by exchangers of known design, and of precoolingthe hot furnace gases prior to subjecting them to electricalprecipitation.

Another object is to provide electrical precipitation apparatus iorcleaning furnace gases that permits rapping of the electrodes withoutdanger of unduly high re-entrainment of precipitated fly ash.

Yet another object is to provide improved electrical precipitationequipment that is simple in design and economical to manufacture andoperate, that has a long service life, and that can be operatedcontinuously over extended periods of time Without requiring periodicshutdowns.

In a characteristic embodiment, the device of the invention comprises,in an electrical precipitation apparatus, at least three electricalprecipitation chambers each including spaced electrode members, a commonplenum chamber from which gas to be treated is supplied to saidprecipitation chambers, separate gas passages leading from said plenumchamber to each of said precipitation chambers, means for selectivelyand separately interrupting the flow of gas between Said plenum chamberand each of said precipitation chambers, and means intermediate saidplenum chamber and the outlet ends of said precipitation chambers toeffect a substantial pressure drop in the flow of gas therethrough.

Air preheaters as commonly supplied on steam generating units may beused advantageously as the means for effecting a substantial pressuredrop in the flow of gas to the precipitation chambers. f

The foregoing and other objects and advantages of the invention will bein part apparent and in part pointed out in the following detaileddescription of a preferred embodiment of the invention as shown in theaccompanying drawing wherein: Fig. 1 is a plan view of an electricalprecipitation system in accordance with the invention, parts beingbroken away to show interior construction, and Fig. 2 is a fragmentaryplan view of another form of electrical precipitation system inaccordance with the invention.

Referring to the drawing, particularly to Fig. 1 thereof, the electricalprecipitation equipment shown is especially suited to the removal of iyash from furnace gases. Hot furnace gases enter the apparatus through aninlet duct I in the direction of the arrow at the top of the sheet andow through the apparatus to an outlet duct 2 under draft forced from theinlet side or induced from the outlet side by suitable fans or blowerswhich may be assisted by stack draft. These devices, well known to theart, are not illustrated.

From the inlet duct, the stream of hot gases passes to a plenum chamber3 and from the plenum chamber through four passageways 4, 5, 6 and 1,defined by the casing 8 and the three parallel gas impervious plates 9.After issuing from the four passageways, the streams of gas are reunitedin the chamber I5 and pass out of the apparatus.

As illustrated in the passageway 4, there is provided in each of thepassageways a precipitating chamber II having a plurality ofprecipitating zones I2, I3 and I4 arranged serially. Each precipitatingzone is energized in conventional manner and includes the collectingplate electrodes I5 and the complementary wire or rod dischargeelectrodes I5. Preferably, the plates I5, the partitions 9, and thewalls of the casing 8 are of metallic construction and are grounded; andthe discharge electrodes I5 are insulatedly supported in parallelcurtains between the collecting electrodes and are supplied with hightension current. As will be understood by persons skilled in the art towhich this invention pertains, when the precipitator is energized,ionizing discharge emanates from the wires to charge particles suspendedin the gas stream which are then attracted to and precipitated upon thecollecting plates I5, the walls of the casing 8, and the partitions 9.

Extending transversely of the passageways 4, 5, 6 and 'I in a zoneintermediate the plenum chamber 3 and the precipitation chambers II is aheat exchanger including the pipes I? provided with annular ns I8 forincreasing the area of heat exchange contact between the finned pipesand the stream of hot gases T owing thereover. The finned pipes I? aresupported in header plates I9 and 2li and pass through apertures in thepartitions 9. Although only the top layer of finned pipes appears in thedrawing, it will be understood that the layers of pipes are arranged intiers extending vertically from top to bottom of the passageways 4, 5, 5and I. The specific showing of the heat exchanger is merely illustrativeand it will be understood that other types of air heaters that arecommercially available may be substituted for the heat exchanger shown,provided the air heater is modied to provide a number of hot gaspassages corresponding to the number of precipitating chambers.

Cold air to be used as combustion air for the furnace enters the heatexchanger through the inlet pipe 2I and passes through the finned pipesI'I to the pipe 22 through which it is led to the furnace. In passingthrough the heat exchanger, the cold air is heated by the combustiongases owing over the finned pipes and returns to the furnace as sensibleheat' much of the heat which would otherwise be lost. The combustiongases are cooled in the process and are delivered to the precipitationchambers at a low temperature permitting the use of low heat-resistantmaterials in the construction of the precipitation chambers, minimizingcorrosion therein and reducing the volume of gas flowing through theprecipitation chambers.

The bank of nned pipes I'I provides in each of the passageways meanseffecting a substantial pressure drop therethrough. Specifically, thetotal resistance to gas flow through one of the passageways 4, 5, 6 and1, including a portion of the heat exchanger or other resistance means,should be about two to five times the resistance of the passageway inthe absence of the heat exchanger section, such resistances beingmeasured in the range of gas velocities encountered in normal operation.The back pressure drop through the heat exchanger may range from 2" to 5of water, for example.

At the entrance to the passageways 4, 5, 6 and 'I there are provided thedampers or louvers 23, 2li, 25, and 25 that are selectively andseparately operable as units by conventional means (not shown) to closeor open any one or more of the passageways. Dampers 24 are shown inclosed position and dampers 23, 25, and 26 are shown in open position.The dampers may 'be set to intermediate positions, if desired.

In the absence of 'the pressure drop imposed by the heat exchanger, gasflowing through the apparatus would tend to favor passageways 4 and 6when the passageway 5 is blanked off, and the passageway 'I would carryless than its proportional load. Such a condition would tend tore-entrain precipitated dust from the precipitation chambers ofpassageways 4 and 6 with resultant loss in efficiency and pollution ofthe ambient atmosphere. However, with the heat exchanger included, theadded resistance thereof distributes the main gas stream substantiallyuniformly to the passageways 4, 6, and I when passageway 5 is closed forrapping or servicing.

The difficulties encountered in the absence of the construction of thepresent invention are even more pronounced when one of the outerpassageways 4 or 'i is closed; conversely, the advantages of utilizingthe principles of the invention are even more evident. Were passageway 4to be closed and the others left open, passageway 5 would carry aninordinate load where no resistance means is provided. On the otherhand, with the heat exchanger forming a component of the apparatus, goodgas distribution between passageways 5, 5, and 'I is obtained under suchcircumstances.

It may be pointed out that, in the exemplary construction shown in thedrawing, those portions of the partitions 9 that lie on ythe downstreamside of the heat exchanger and serve to separate the precipitationchambers I I from each other' may be omitted without losing theadvantages of the invention. In such case, the remaining portions of thepartitions 9 serve to direct the gas streams into the severalprecipitating zones El, and, once the path of flow has been establishedthe gas will continue to iiow rectilinearly through the precipitatorswithout diffusing to an adjacent dampered zone.

The advantages of the invention may be realized. where .means providinga high pressure drop are placed at the outlet ends of the precipitationlchambers on the upstream .side of the chamber I0, for example. Theresistance means may take the form of cyclone separators arranged inparallel with each other, one or more separators being in series witheach precipitation channel. This construction is shown in Fig. 2 whereinparts similar to the parts of the system of Fig. l are indicated bysimilar but primed reference numerals.

Referring to Fig. 2 the precipitating passageways or chambers Il', 5', Gand 'l' are arranged in parallel and in each are positioned the usualcomplementary discharge electrodes le and collecting electrodes l5. Thegas outlet 'ends of the precipitation chambers communicate with theoutlet chamber le and outlet duct 2. Adjacent the outlet end of each ofthe precipitation chambers is a cyclone separator indicateddiagrammatically by the numeral 2:?. These cyclone separators arearranged each in series with one of the'precipitation chambers and inparallel with each other. The separators provide the desired resistanceto gas flow through the open4 precipitation chambers and eiectredistribution of 'fas flow to the open chambers when one or more of thechambers is closed for cleaning and the like.

The apparatus of the invention includes at least three precipitationchannels in parallel, so that when one channel is blanked ofi, there areat least two channels remaining open to carry their respective andproportionate shares of the gas stream.

I claim:

1. In electrical precipitation apparatus, a plurality of electricalprecipitation chambers each including spaced complementary electrodemembers, a common plenum chamber from which gas to be treated issupplied to said precipitation chambers, separate gas passages leading.from said plenum chamber to each of said precipitation chambers, heatinterchange means in each of said gas passages extending transversely ofsaid gas passages and spaced in the direction of gas flow from saidelectrode members to afford a substantial resistance to the flow of gasthrough said passages, and means in each of said gas passages forselectively and separately interrupting the ow of gas between saidplenum chamber and each of said precipitation chambers.

2. In electrical precipitation apparatus for removing ly ash fromfurnace gases, at least three electrical precipitation chambers eachincluding spaced complementary electrode members, a common plenumchamber from which hot furnace gases to be treated are supplied to saidprecipita-- tion chambers, separate gas passages leading from saidplenum chamber to each of said precipitation chambers, heat interchangemeans in each of said gas passages extending transversely of said gaspassages and spaced in the direction of gas oW from said electrodemembers to afford a substantial resistance to the ow of gas through saidpassages and means in each of said gas passages for selectively andseparately interrupting the ow of gas between said plenum chamber andeach of said precipitation chambers.

3. Apparatus for treating hot furnace gases containing y ash to cool andclean the same comprising means providing an elongated casing, meansadmitting furnace gases to said casing, a plenum chamber in said casingreceiving said furnace gases, spaced parallel partitions in said casingextending longitudinally from the exit end of said plenum chamber to anoutlet chamber in said casingA and dividing said casing into 'at leastthree'passages in parallel, louver means at the entrance ends of saidpassages for selectively and separately interrupting the cw of gasbetween said plenum chamber and said passages, heat interchange meanshaving a plurality of finned pipes extending continuously andtransversely across said passages on the downstream side of said louvermeans, said heat interchange means providing a substantial 'resistanceto the flow of gas through said passages, electrical precipitation meansin said passages on the downstream side of said heat interchange means,and gas outlet means leading from said outlet chainber.

Il. A gas treating apparatus comprising a casing, gas inlet meanslocated at one end of said casing, gas outlet means located at the otherend of said casing, said casing housing in spacev relationship in thedirection of gas ilow; louvre means, heat interchange means electricalprecipitator means, and dividing-walls in said casing extending in thedirection of gas flow and sectionalizing said louvre means, said heatinterchange means and said electrical precipitator means into aplurality of parallel sections, said heat interchange means extendingtransversely of said passages and substantially equalizing gas flowthrough the operating sections by providing a substantial resistance tothe flow of gas in the operating sections when gas flow is interruptedin one of the sections by the louvres.

5. Electrical precipitation apparatus comprising a plurality ofelectrical precipitation chambers each including spaced complementaryeleotrode members, a common plenum chamber from which gas to be treatedis supplied to said precipitation chambers, separate gas passagesleading from said plenum chamber to each of said precipitation chambers,heat interchange means in each of said gas passages extendingtransversely of said gas passages and spaced in the direction of gas nowfrom said electrode members to afford a substantial resistance to theflow of gas through said passages, and means for selectively andseparately interrupting the flow of gas between said plenum chamber andeach of said precipitation chambers.

6. Electrical precipitation apparatus comprising at least threeelectrical precipitation chambers arranged for parallel ow of gastherethrough, each of said chambers having spaced complementaryelectrode members therein, gas inlet and gas outlet means communicatingwith each of said chambers, a common plenum chamber communicating withthe gas inlet means of said electrical precipitation chambers, gas inletmeans communicating with said plenum chamber, means selectively andindependently blocking and unblocking flow of gas through each of saidprecipitation chambers, and xed gas ow resistance means positionedadjacent at least one end of each of said precipitation chambers, spacedin the direction of gas ow from said electrode members and imposingsubstantial resistance to the flow of gas through said precipitationchambers.

7. Electrical precipitation apparatus comprising at least threeelectrical precipitation chambers arranged for parallel now of gastherethrough, each of said chambers having spaced complementaryelectrode members therein, gas inlet and gas outlet means communicatingwith each of said chambers, a common plenum chamber communicating withthe gas inlet means of said electrical precipitation chambers, gas inletmeans communicating with said plenum chamber, means selectively andindependently blocking and unblocking flow of gas through each of saidprecipitation chambers, and heat interchange means positioned adjacentthe inlet end of each of said precipitation chambers, spaced in thedirection of gas flow from said electrode members and imposingsubstantial resistance to the iloW of gas through said precipitationchambers.

8. Electrical precipitation apparatus comprising at least threeelectrical precipitation chambers arranged for parallel ilow of gastherethrough, each of said chambers having spaced complementaryelectrode members therein, gas inlet and gas outlet means communicatingwith each of said chambers, a common plenum chamber communicating withthe gas inlet means of said electrical precipitation chambers, gas inletmeans communicating With said plenum chamber, means selectively andindependently blocking and unblocking flow of gas through each of saidprecipitation chambers, cyclone separator means positioned adjacent theoutlet end of each of said precipitation chambers, spaced in thedirection of gas flow from said electrode members and imposingsubstantial resistance to the flow of gas through said precipitationchambers.

j 9. Electrical precipitation apparatus as dened in claim 6 wherein theresistance to gas flow through said gas ow resistance means is fromabout two to above five times the resistance to gas flow through saidprecipitation chambers.

CARL W. J. HEDBERG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 0 2,184,782 Scott et al Dec. 26,1939 FOREIGN PATENTS Number Country Date 339,625 Germany July 29, 1921323,186 Great Britain Dec. 23, 1929

